The invention relates to semiconductor laser and photo detecting arrays for wavelength division multiplexing interconnection system.
Implementation of wavelength division multiplexing optical interconnections are very important for high speed and high density optical transmission and processing. The value and importance in development of the semiconductor optical devices necessary for wavelength division multiplexing optical interconnections are on the increase as the requirement of the realization for the wavelength division multiplexing optical interconnections are on the increase. The use of the wavelength division multiplexing permits for a great amount of optical informations to be transmitted through a single or a small number of optical fibers or optical wave guides for high speed and high density processing and transmission of the optical informations. Such the optical interconnections requires a large number of laser and photo detector devices which are for optical waves having different wavelengths to each other. For that purpose, various types of wavelength tunable semiconductor lasers and filters has been on the development. Conventionally, the developments of edge emitting laser devices such as the distributed feedback lasers and the distributed Bragg reflector lasers have been influential. The use of such the edge emitting laser devices for wavelength division multiplexing optical interconnections has have disadvantages in the difficulty in the device fabrications and a relatively narrow tunable wavelength band width, for example, approximately several ten angstroms only.
On the other hand, vertical to surface emitting lasers are more attractive than the above edge emitting laser devices as being suitable for two dimensional integration or two dimensional arrays for an achievement of multiple channels with a large number of different wavelengths. The two dimensional arrays comprising a great number of the surface emitting lasers for emitting laser beams with different wavelengths may be formed in a single wafer to form the two dimensional arrays. It is disclosed in IEEE Quantum Electronics vol. 27, No. 6 pp. 1368-1376 to align the two dimensional arrays of the surface emitting lasers including distributed Bragg reflector mirrors comprising alternate large band gap and small band gap semiconductor layers whose thickness is gradient across the wafer. Since the vertical cavity surface emitting laser wavelength depends sensitively on the cavity length, the gradient of the thickness of each of the alternate large band gap and small band gap semiconductor layers forming the distributed Bragg reflector mirrors involved in the vertical cavity surface emitting laser. One way to generate such thickness variation is to keep the wafer stationary during part of the molecular beam epitaxy growth. The thickness variation is caused by the variation of the amount of atoms arriving at the wafer surface in the direction parallel to the plane of incidence of the source.
As the photo detector, double vertical-cavity detectors are suitable as having a sensitivity in a relatively wide range wavelength. It is disclosed in Japan Journal of Applied Physics vol. 32, pp. 600-603, Part 1, No. 1B, January 1993 to use double vertical cavity photo detector and single vertical cavity laser sections for two dimensional bi-directional optical interconnections. The vertical cavity structure involved in each the laser and detector sections has an absorption layer sandwiched between top and bottom distributed Bragg reflector mirrors each of which comprises alternate large band gap and small band gap semiconductor layers having a thickness of a quarter of the wavelength. Further, the double vertical cavity structure involved in the photo detector section has a spacer layer in the bottom distributed Bragg reflector mirror in which the spacer layer has a thickness of a half of the wavelength. The existence of the spacer layer permits the photo detector to show a sensitivity in the wide range of the wavelength.
The serious problem with the prior arts are in the difficulty in application for the wavelength division multiplexing optical interconnections. The realization of the wavelength division multiplexing optical interconnections between the two dimensional laser and photo detector arrays necessarily requires the exact correspondence between the wavelength of the laser beam emitted from individual lasers in the arrays and the wavelength of corresponding photo detectors in the arrays. The wavelengths of both the vertical cavity surface emitting laser and the double vertical cavity photo detector are extremely sensitive to a slight variation of the thickness of any layers constituting the laser or detector device. Actually, it would extremely be difficult to implement such a precise control of conditions for device fabrications as to suppress any variation of the thicknesses of any layers constituting the laser or photo detector device. Particularly, the strict control of the thickness of the layers constituting the distributed Bragg reflectors are very important as described above. The above layers involved in the photo detector and laser devices are grown by molecular beam epitaxy. When the layers constituting the surface emitting laser device and the layers constituting the photo detector device are grown in different or not successive fabrication steps by molecular beam epitaxy, a variation in the thicknesses of the layers between the laser and photo detector sections necessarily appears as the conditions in the growth of the layers by molecular beam epitaxy are unavoidably varied between the different or non-successive growth steps. Even if such the variation of the conditions for the growth of the layers in the different or non-successive growth steps by the molecular beam epitaxy is extremely slight, it is no longer possible to obtain the required exact correspondence in the wavelengths between the laser device and the photo detector device. The laser device and the photo detector device are respectively different wafers, two non-successive growths are necessary for the laser device and the photo detector device respectively. This results in a difficult in obtaining the correspondence in the wavelength between the laser device and the photo detector device which are paired to each other.
It is therefore required to grow the layers for the laser device and the photo detector device in successive growth steps to obtain the exact correspondence in the wavelengths between the laser device and the photo detector device.